One downside of being our household’s resident “Mr Scott” (ie the “miracle worker” who can solve any tech/IT-related problem that arises): when you find issue(s), which have no obvious cause, but they still exist, and need fixing.
This morning, there were multiple problems in our house, all concerning network access. I gradually fixed all but one (not always sure *how*…), and I think I just solved the last one.
TL;DR: if your TP-Link Deco app can’t connect to the Deco, un-/re-install the app.